The fruitless locus in the fruit fly Drosophila melanogaster will be analyzed genetically, incorporating a variety of molecular and anatomical techniques. By prior analysis, this locus controls a variety of male- specific, sex-behavior related functions; its function at the level of gene expression, however, is completely uncharacterized. The fruitless function that will be studied here is the role this locus plays in the regulation of the development of a large muscle that occurs specifically in adult male abdomens. One approach will be to induce and isolate new mutations that upset this function; the "mutagenic agent" will be a marked transposon which will facilitate direct cloning of fruitless DNA contiguous with the insert. This could lead to cloning and molecular characterization of fruitless. An independent set of experiments will assess the phenotypic impact of these new mutations on neuromuscular anatomy; the abdominal musculature and its innervation will be characterized in both mutant and normal muscles. A correlating experiment will similarly assess male mosaics whose abdomens are part normal, part fruitless in genotype. This experiment could potentially determine whether fruitless encodes a "developmental inducer" which regulates the development of this muscle. These experiments will expose students to genetic logic, and teach the utility of genetic variants in deducing gene function underlying the development of the male-specific muscle.